1. Field of the Invention
The present invention relates to a metal halide lamp used in general purpose illumination, optical appliances and others, an illumination optical apparatus combining a metal halide lamp and a concave reflector, and an image display system such as projection type liquid crystal display.
2. Description of the Prior Art
Recently, the metal halide lamp has been widely applied for lighting at shops, roads, and for other general purposes, and its demand is also spreading as lights for automobiles, or light source for optical appliances. An example of metal halide discharge lamp is shown below while referring to drawings.
FIG. 1 shows a structure of a single tube type metal halide lamp. In FIG. 1, numeral 1 denotes a luminous part of a discharge tube made of quartz glass, 2 is a tungsten electrode installed through a molybdenum foil 10, 3 is a sealing part tightly adhering the molybdenum foil 10, and 4 is an external lead wire.
In the metal halide lamp composed of these constituent elements, the operation is described below.
In the metal halide lamp, the metal halide added in the discharge tube together with mercury and rare gases is melted and is present as liquid phase while the lamp is lighting at the inside wall of the discharge tube. The liquid metal halide is evaporated to be gas phase, and the metal halide vapor is dissociated into metal atoms and halogen atoms in the high temperature region of the arc column. The metal atoms are excited by the arc and emit their own characteristic spectral lines. Accordingly, as compared with the high pressure mercury lamps, the metal halide lamp is superior in luminous efficacy and color rendering properties. Metal halide lamps containing metal iodides such as Tl--Na--In, Sc--Na, Dy--Tl, and Dy--Nd--Cs are widely put in practical use.
Generally, in the metal halide lamps, the luminous characteristic of the lamp is determined by the vapor pressure of the metal halides inside. So it is necessary to let the coolest-spot temperature of the discharge tube high enough to increase the vapor pressure of the metal halides in order to obtain the luminous characteristic of the metal halide additives. For this purpose, in the metal halide lamp, the tube wall load (electric power/all inner wall area) is appropriately designed to obtain the desired coolest-spot temperature. Moreover, the heat-reflecting coating is usually is applied on the outer surface of the coolest spot of the discharge tube.
In such conventional metal halide lamps, however, when the temperature of the coolest spot is raised to heighten the vapor pressure of the added metal halides and to improve the color rendering properties, the metal additives and constituent material of the discharge tube react rapidly resulting in the shortening of life caused by the drop of luminous flux or the rupture of the discharge tube.
It is known that color separation phenomenon of the arc occurs in the metal halide lamps. This phenomenon is resulted from the dependence of the emission spectrum and color on the arc position. The arc temperature is not uniform over the entire arc but is different depending on the arc position, so that the excitation state of atom or the emission from atomic species is different. The arc temperature is highest on the arc axis between two electrodes, and the arc temperature declines from the arc center to the inner wall direction of the discharge tube. For example, in the metal halide lamp filled with rare earth iodides DyI.sub.3, NdI.sub.3, CsI, the emissions of mercury ions and neutral atoms of Dy and Nd with a large excitation energy, are dominant in the high temperature region of the arc column. In the outer region with lower temperature, light emission from neutral atoms of Dy and Nd become dominant, and in the outermost area with even lower temperature, Cs and DyI molecules emit light mainly.
When such metal halide lamp is arranged as shown in FIG. 2 so that the arc axis may be positioned on the optical axis of the concave reflector 5 to compose an illumination optical apparatus, the color distribution of the screen 6 of an image display system is affected by the color distribution of the arc. That is, the center of the screen corresponds to the central axis of the metal halide arc, and the edge area of the screen corresponds to the outer region of the arc. The color distribution and spectral distribution of the arc from the central axis to the arc periphery correspond to the color distribution of the screen from the center toward the edge. Therefore, if the arc color separation phenomenon as mentioned above occurs in the metal halide lamp light source, there will be a large ununiformity of spectral distribution, or the color in the center and periphery of the screen. When such metal halide lamp filled with DyI.sub.3, NdI.sub.3, CsI is used in an illumination optical system as a light source, the screen center area of the image display system tends to be greenish and the color temperature is high, while the peripheral area of the screen tends to be reddish and the color temperature is low.
Conventionally, a technique is used to improve the uniformity of arc color by processing the outer surface of the discharge tube of the metal halide lamp in an opaque state (ground glass state) by sand blasting or similar method (hereinafter called frost processing). However, in such an optical system where the light emitted from the lamp is condensed by a concave reflector, frost processing of the outer surface of the lamp causes the apparent size of the light source to increase and therefore lowers the efficacy of the reflector. Even if the color uniformity of the screen is improved, the brightness of the screen is lowered.